There is now substantial evidence to indicate that the pineal gland, melatonin, and a biological clock are all involved in the photoperiodic control of seasonal reproductive cycles in mammals. However, at the present time there is not a coherent picture as to how these factors are integrated with the neuroendocrine-gonadal axis. Our recent studies indicate that melatonin treatment does not directly alter the hypothalamic-pituitary-gonadal axis, but instead interferes with the ability of the animal to respond to photoperiodic information. Experiments will be carried out under a variety of different light: dark cycles and at different stages of the annual reproductive cycle to test this hypothesis. In examining the relationship between the circadian system and the neuroendocrine-gonadal axis, we plan to stimulate the suprachiasmatic nuclei of the hypothalamus and determine if circadian or reproductive function have been altered. Since the photoperiod is known to alter the sensitivity of gonadotropin control centers to the negative feedback effects of steroid hormones, we plan to determine if melatonin mediates this pineal-dependent response. The overall objective of this research program is to determine how the pineal gland and the circadian system are involved in the regulation of neuroendocrine-gonadal function in mammals.